This invention relates to a configuration of a combustion chamber in a direct injection spark ignition engine.
Tokkai 2000-087750 published by the Japanese Patent Office in 2000 discloses a direct injection spark ignition engine which performs stratified combustion in a predetermined operating region. In the stratified combustion operating region of this engine, the spray of fuel which is directly injected into a combustion chamber from a fuel injector is guided to the periphery of a spark plug using the circulation of air formed in a longitudinal direction inside the combustion chamber. As a result, a mass of air-fuel mixture formed on the periphery of the spark plug is ignited by the spark plug. In the following description, the circulation of air in the longitudinal direction will be referred to as xe2x80x9ctumblexe2x80x9d.
In order to form tumble inside the combustion chamber, the prior art engine comprises a tumble generating valve for blocking the lower half of an intake passage. The tumble generating valve is one type of a so-called charge motion control valve (CMCV). When the tumble generating valve is closed in the stratified combustion operating region, the velocity of flow of the air which is aspirated into the combustion chamber from the intake port via an intake valve increases, thereby generating tumble in the combustion chamber. When the fuel injector injects fuel toward this tumble, a mass of air-fuel mixture is produced on the periphery of the spark plug as a result of the gas flow created by the tumble.
Tokkai Hei 10-008968 published in 1998 and Tokkai Hei 11-324679 published in 1999 by the Japanese Patent Office propose that a cavity formed on of the piston be aligned with the curve of the tumble such that a circular tumble having an equal diameter to the cylinder bore is generated in the combustion chamber when the piston is at its bottom dead center.
Tumble with an equal diameter to the cylinder bore can only exist in the central portion of the cylinder, where the width of the longitudinal cross section of the cylinder is greatest. The width of the longitudinal section of the cylinder gradually decreases in size as it becomes offset from the central portion, and therefore the tumble generated in such a region attenuates due to interference with the wall surfaces of the cylinder. Consequently, strong tumble cannot be formed using the tumble setting of the prior art.
It is therefore an object of this invention to strengthen the tumble generating function of a direct injection spark ignition engine which performs stratified combustion.
In order to achieve the above object, this invention provides a direct injection spark ignition engine, comprising a cylinder in which a combustion chamber is formed, an intake port for supplying air to the combustion chamber, and a piston which compresses the combustion chamber by performing a reciprocal motion inside the cylinder between top dead center and bottom dead center.
The piston has a cavity for producing a tumble in air that is aspirated into the combustion chamber from the intake port as the piston slides toward the bottom dead center. The cavity comprises a bottom surface which is curved to conform to a predetermined tumble which is set in the form of an ellipse having a major axis in the sliding direction of the piston, inside the combustion chamber when the piston is at its bottom dead center.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.